This invention relates to a diffraction grating and to an optical head device which comprises a diffraction grating and is for use in recording optical information on an optical recording medium and/or playing back the optical information from the optical recording medium in an optical disk device which is, for example, a compact disk (CD) device. The optical recording medium may be an optical disk, a digital audio disk, or a video disk. The optical head device is alternatively referred to, briefly, as an optical head.
In the manner described in an elder patent application filed by Yuzo ONO, the present applicant, et al (in the U.S., filed Dec. 10, 1986, as Pat. Application Serial No. 940,007 and in EPC, filed the 9th Dec. 1986 as Pat. Application No. 86 117 152.8), such an optical head device is used in combination with an optical source and an optical detector assembly in dealing with the optical recording medium and has a main optical axis. The optical source is ordinarily a semiconductor laser and is for producing a laser beam along the optical axis. Usually, the optical detector assembly is disposed adjacent to the optical source and is a photodiode which is partitioned into a plurality of individual optical detectors, such as four or six optical detectors.
Generally speaking, the optical head device comprises an optical system which, in turn, comprises a focussing lens and a beam splitter unit. Naturally, the focussing lens has a lens axis which may, or may not, be collinear with the main optical axis as will become clear below. In any event, the optical recording medium is held perpendicular to the lens axis. Responsive to an input or incident beam received along the lens axis, the focussing lens produces a converging beam which is focussed on the optical recording medium. When used in reproducing the optical information, the optical head device makes use of a diverging beam which is reflected from the optical recording medium as a reflected beam along the lens axis. Responsive to the reflected beam, the focussing lens produces an output, or exit, beam along the lens axis. The beam splitter unit causes the coherent beam to pass therethrough as the input beam and directs the output beam to the optical detector assembly as a plurality of sidewards directed beams along side optical axes which customarily are not coincident with the main optical axis. There may be two sidewards directed beams, in which case there would be two side optical axes are.
In the optical head device disclosed in the elder patent application, the beam splitter unit is uniquely implemented by a diffraction grating which has a plurality of grating regions. Responsive to the laser beam, the grating regions cooperatively produce a zero-order diffracted beam as the input beam. Responsive to the output beam, the grating regions individually produce sidewards diffracted beams as the respective sidewards directed beams. Each sidewards diffracted beam is preferably a first-order diffracted beam.
Regarding the optical head device revealed in the elder patent application it should be noted that the coherent beam passes through the diffraction grating as the zero-order diffracted beam on its way to the focussing lens and, preferably, as the first-order diffracted beam returning from the focussing lens. The diffraction grating therefore has a poor laser-beam utilization efficiency which can be defined as a product of a diffraction efficiency of the diffraction grating for the zero-order diffracted beam and another diffraction efficiency for the first-order diffracted beam. The laser-beam utilization efficiency is at most 10% when the diffraction grating is a ridge and groove grating.
Furthermore, it should be noted that the diffraction grating described above is not much different from a single diffraction grating which is used in various optical devices, such as a spectrometer. Additionally, the laser beam is not necessarily a linearly or plane polarized beam.